Modern wind turbine blades are typically constructed from two half shells: a windward shell and a leeward shell. The half shells are formed in separate mould halves of a mould assembly. After forming the half shells in their respective mould halves, adhesive is applied along leading and trailing edges of one or both half shells. One mould half is then lifted, turned and positioned on top of the other mould half. This is known as closing the mould, and results in the two half shells being positioned in contact along their respective leading and trailing edges. The adhesive between the two half shells is then cured, which results in the half shells being firmly bonded together to form the complete blade.
The wind turbine blade typically includes one or more shear webs located inside the blade and bonded between inner surfaces of the windward and leeward half shells. The shear webs are longitudinal structures, which when viewed in cross-section are substantially I-shaped or C-shaped and comprise a web having first and second flanges at its respective ends. The flanges are bonded respectively to the inner surfaces of the respective half shells and the web thereby forms a bridge between the half shells.
Prior to closing the mould to bond the half shells together, a longitudinal strip of adhesive is applied to the inner surface of one of the half shells and the shear web is positioned on top of the adhesive. The shear web is typically arranged vertically such that a lowermost flange of the shear web sits on top of the adhesive. A further strip of adhesive is applied along the uppermost flange of the shear web. If the blade has multiple shear webs then this process is repeated for each shear web. The mould is then closed. The adhesive between the flanges of the shear web(s) and the respective inner surfaces of the blade shells is compressed when the mould is closed and cures at the same time as the adhesive between the half shells.
It is important to ensure sufficient squeezing of adhesive between the shear webs and the respective half shells during mould closure in order to form consistent bond lines between the flanges of the shear webs and the inner surfaces of the respective half shells. One way of achieving this is described in the applicant's prior PCT application WO2008/104174, whereby the shear web incorporates a resilient member arranged to compress during mould closure and subsequently expand during curing of the adhesive. This results in the flanges of the shear web being forced towards the respective inner surfaces of the half shells such that they exert a compressive force on the adhesive during curing.
The shear web described in WO2008/104174 is illustrated in FIG. 1. Referring to FIG. 1, the shear web 10 is formed in multiple parts and comprises a web panel 12 and a web head 14 fitted over an upper longitudinal edge 16 of the panel 12. The web panel 12 is substantially L-shaped and defines a lower flange 18 of the shear web 10 for bonding to an inner surface of a first half shell of the blade. The web head 14 is substantially T-shaped in cross-section and defines an upper flange 20 for bonding to an inner surface of the second half shell of the blade.
The T-shaped web head 14 includes a longitudinal slot 22. A strip of foam 24 is provided at the base of the slot 22, and the slot 22 is filled with adhesive. The upper edge 16 of the panel 12 is then inserted into the slot 22 such that the strip of foam 24 is located between the upper edge 16 of the panel 12 and the base of the slot 22. When the mould is closed to bond the shell halves together, the strip of foam 24 is compressed initially and then subsequently expands slightly to force the web head 14 towards the inner surface of the half shell. Accordingly the foam 24 functions as a spring.
A number of drawbacks have been identified with the above solution. In particular, the strip of foam 24 does not perform particularly effectively as a spring, and hence squeezing of adhesive between the flanges 18, 20 of the shear web 10 and the inner surface of the blade shell is still sub optimal. Further, the strip of foam 24 occupies a critical region at the base of the slot 22 in the web head 14. As the strip of foam 24 extends along the entire length of the web head 14, it prevents adhesive from accumulating at the base of the slot 22. Accordingly, an adhesive void is effectively present at the base of the slot 22, which prevents the longitudinal edge 16 of the panel 12 bonding to the web head 14 at the base of the slot 22.
Another problem identified with the above solution in practice is that it can be difficult to ensure that the web panel 12 is inserted centrally into the slot 22 in the web head 14. Once the mould is closed, it is then not possible to inspect the position of the panel 12 inside the web head 14. If the web panel 12 is not inserted centrally in the slot 22, then it may wipe away adhesive from one side of the slot 22 during insertion. This can result in a sub-optimal bond forming between the web panel 12 and the web head 14 because the panel 12 may only then be bonded on one side of the slot 22.
The present invention aims to overcome one or all of the above problems.